Balanced microwave detector



May 5, 1953 R. A. BRADEN BALANCED MICROWAVE DETECTOR Original Filed Aug. 20, 1945 2 SHEETSSHEET 1 mmme' $575670)? May 5, 1953 R. A. BRADEN BALANCED MICROWAVE DETECTOR Original Filed Aug. 20, 1945 2 SHEETS-SHEET 2 V m my my zzvmvrox. ll ma/115% illlllu I IIIIIIIIIIII \IIIIuuZ Patented May 5, 1953 seesaw ruw avacrrsesme Rene 'A. Braden, Hopewell, N. 41., 'assignor to Radio Coiporation of America, a corporationof Delaw are Ofrig inaLapplication August 20, .1945, .S erial N p.

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.This invention relates generally to microwave receivers land moreparticularly to balanced mitcrowavedetectors-for coupling balanced or dif- 1 ferent input circuitsto'balanced detector output circuits. This is r a divisionalapplication of my copending application, Serial No. 611,646,-en-

titled Balanced Microwave Detector, filed August v2.0, 1945, -.now :Patent No. 2,550,524, granted April 24,: l 95 l.

:Balanced vmicrowave antenna, detector and amplifiericircuits are particularly: advantageous at microwave frequencies for the reasonth-at an :improved ratio of signal-to-noise levels may' be obtained. At microwave-frequencies above 3000 megacycles. diificulties in phasing and matching signal levels' heretofore 1 have prevented effective .use. of balanced. detector and amplifier circuits. The instant invention comprises several embodiments of balanced microwave detectors which have been found tobecompletely satisfactory and; to require. only simple adjustment. Conventional, microwave; crystal detectors or diodes may be employedlsince provision ismade'for matching the .detector impedance to the. associated net .works.

'gBriefly. the: instant invention comprises a pair of microwave crystal or thermionic discharge .tube detectors leach coupled to .a waveguide ;structure or cavity resonator in a manner i-Wherehy the detector may be matchedto the network surge :impedance. Input signals from 1 a 1 balanced input circu-it, such, for example, as 1 a dipole antenna, or from two/circuits operating .atfslightly. different;irequencieaare coupled into the detector waveguides cor resonators from separate lines which are terminated and coupled into the detector networks in a manner providing fieldsin phase opposition. A first embodi- -rnent of the invention employs a single waveguide having two detectorscoupled thereto at its -opposite endsandmatchedthereto by means of reactivetuning screws or similar devices adja- ;cent to each detector. Input signals from the -input-circuits arecoupled into the waveguide enclosure through two coaxial lines which enter the waveguide at a point;equidistantjromand intermediate the two detectors. One line is-terminated .Within it the waveguide in ;a capacitive radiating .element which provides similarlyphased .microwave fields in both sides of the waveguide. The other of the inputlines is terminated in a single couplingloop \vhich provides oppositely-phased fieldsinthe two sides of the waveguide. A modification ofsaid first embodiment, of the invention utilizes coaxial v line prin- .Q Pl, .1 Q pheein l heimwts nals detector employing two cavity resonators len- :The second embodiment v-of the'invention; utilizesl tWoseparate .cavitysresonators. each enplosing one of the-microwave detectors and including means, such as tuning screws or tuningplugs, for

tuning the cavityv resonators separately. Thein- -put signal lines are each terminated in a T-junction. The two arms. of each of the T-junctions are terminated by coupling loops in different. ones of the cavity resonators. minated by similarly-phased coupling loops =;i n

One IT-junction is; terboth of the cavity resonatorsand the other T-junction is terminated byoppositely-phased coupling loops in each of the two resonators.

A modification of the secondembodiment of the invention employs a lossy line for ,the two arms of one of 1 the IT-junction wherebyrreaction between the cavity resonators is efiectively prevented. The lossy T-junction line may comprise -a section of coaxial line having a lossy dielectric such-for example, as conductive rubber.

.Among the objects of the invention are:to-.-pr.ovide an improved method of and means forlde- 'tecting-microwave signals in a balanced detector circuit. -A further object is tmproVidean-improved balanced microwave detector for frequencies in excess of 3000 megacycles. A-njadditional object is to provide abala-nced microwave .detector comprising a waveguide network termitnatedat each end by a microwave detector and having input lines terminated-in said :Waveguide equidistant from and intermediate said detectors by elements providing oppositely-phased electric "fields within the two portions =0f-.said waveguide adjacent said detectors. A'iurther-object' is {to provide means for matching the microwave detectors insuch a waveguidecouplingnetwork to the surge impedance of @said Waveguide. An additional object is to providea; balanced; micro- .wave detector utilizing a coaxiallinefor deriv- 'ing the desired phasing of input isignalstoabe detected.

Another object is-to providea balanced-microresonators by means of similarly phased and oppositely-phased coupling -;elements. An additionalobject is to provide a balanced microwave closing separate microwave detectors, and having input lines each terminated in T-junctions both of which arecoupled cophasally and in opr-junctions effectively to prevent reaction between said cavity resonators.

The invention will be described in greater detail by reference to the accompanying drawing in which Fig. l is a schematic layout of one embodiment of the invention, Fig. 2 is a schematic layout of a modification of said first embodiment of the invention, Fig. 3 is a schematic layout of a second embodiment of the invention, and Fig. 1 is a schematic layout of a modification of said second embodiment of the invention. Similar reference characters are applied to similar elements throughout the drawing.

Referring to Fig. 1, a pair of input signal coaxial lines 3 have their inner conductors 5, terminated in a capacitive plate 9 and a coupling loop II, respectively, within the center portion of waveguide section l3. The capacitive plate 9 excites similarly-phased electric fields M in both ends of the waveguide section |3. The coupling loop excites oppositely-phased fields |6, |8 in both ends of the waveguide section l3.

This arrangement may be used for all the purposes for which balanced detectors are used. For example, input A might be a receiving antenna, and input B a local or beating oscillator (or vice versa). Suitable matching means would be provided for A, but not necessarily for B. By reason of the opposing polarities of the two detectors with respect to the output transformer, there is no output signal corresponding to amplitude modulation of either signal. Thus, noise,

. and power supply ripple modulation of the local oscillator are balanced out. However, because of the reversed radio frequency phase at one detector compared with the other, the heterodyne output component of one detector is reversed and the two detected signals add instead of balancing out. Thus a pure beat signal is obtained, free from noise which modulates the local oscillator. Also, due to the phasing of the two input coupling means, cross-coupling between the two input circuits is minimized, permitting either or both input circuits to be matched to the waveguide without objectionable loading of one input circuit by the other.

A first microwave detector I is inserted within, and coupled to, the waveguide section l3 adjacent one end thereof. The microwave detector l5 may comprise a microwave crystal of the type described in the copending application of Wendell L. Carlson, Serial No. 507,755, filed October 26, 1943, Patent 2,427,087, or it may be any type of thermionic discharge tube or other microwave-responsive device known in the art. The detector |5 may be matched to the surge impedance of the waveguide section |3 by means of tuning screws |9 disposed adjacent thereto intermediate the detector and the center portion of the waveguide section. Detected signals derived from the detector |5 may be coupled to an external circuit by a conductor which is by-passed by a capacitive coupling 2| through the wall of the waveguide. One terminal 23 of the detector I5 is grounded to the inner surface of the waveguide, and the other terminal 25 passes through an aperture in the waveguide wall and includes capacitive electrodes 21, 29 which are disposed closely adjacent to the waveguide walls forming said aperture to provide the bypass element 2|.

A second microwave detector 3|, of identical type to the detector i5, is disposed adjacent to the opposite end of the waveguide section |3. It

includes a grounded connection 33 and an ungrounded connection 35 which passes through a second capacitive by-pass element 31 to a second external circuit. The two output conductors 25 and 35 may be connected to any type of balanced detector output circuit such, for example, as the input circuits of an intermediate frequency or audio frequency amplifier. The waveguide |3 should be proportioned to provide eflicient transmission at the received microwave frequency. Tuning screws or tuning plugs 39, 4| may be employed to match the second microwave detector 3| to the surge impedance of the waveguide l3.

Exact matching of each detector to the waveguide surge impedance is necessary so that no microwave energy will be reflected from one end of the waveguide section to the other end. Reflected signals would disturb the phase relations provided by the capacitive and inductive input signal couplings described heretofore.

It should be understood that a similar arrangement may utilize a single coaxial transmission line in place of the waveguide structure described heretofore, and that the coaxial line may be tuned by tuning stubs or other devices known in the art.

The matching adjustments for the detectors are preferably made on one detector at a time, the other detector being replaced temporarily with a non-reflecting load such, for example, as a tapered lossy dielectric plug inserted into a matching waveguide which may be connected at the planes Z-Z or mm to the section of the waveguide l3 to be adjusted. The waveguide should be of telescopic design permitting it to be taken apart at these planes. After each detector has been matched by suitable adjustment of the tuning screws adjacent thereto, the two matched sections of the waveguide are connected together. Preferably the matched waveguide sections connected at the planes lZ or 111-111. during the matching procedure should include wave reflection measuring elements such, for example, as wave detectors connected to movable probes longitudinally adjustable within slots therein.

Fig. 2 illustrates a fully coaxial line modification of the device described by reference to Fig. 1. Similar elements adapted to coaxial line technique are indicated by primed reference numerals corresponding to elements for the same purpose in Fig. 1.

Fig. 3 employs two input signal coaxial lines I and 3, which are terminated in line T- junctions 43, 45, respectively. The end of the line T-junction 43 is terminated in two similarly-phased coupling loops 41, 49, which extend into two cavity resonators 5|, 53 respectively. The line T-junction 45 is terminated in a pair of oppositely-phased coupling loops 55, 51 which extend respectively into the cavity resonators 5| and 53. The first cavity resonator 5| may be tuned, for example, by means of a tuning screw 59. The second cavity resonator 53 may be tuned, for example, by means of a second tuning screw 6|.

The first microwave detector |5, of any of the types well known in the art, or as described heretofore, is enclosed within the first cavity resonator 5|, and is matched by a first tunable line stub 63. Similarly the second microwave detector 3|, of similar type, is enclosed within the second cavity resonator 53, and is matched by a second tunable line stub 65. Signals from the first input line l are coupled into both cavity resonators 52,. 53 in the same phase. Signals from the second input line 3. are coupled into both cavity resonators 5!, 53 in opposite phase. If the input coupling loops t l, and 55, 5? may be of different sizes. The microwave detectors iii and 3! may include capacitive loypass elements 2i and 31, respectively, in their output leads in the same manner described heretofore for the device of l.

A modification of the second embodiment of the invention is illustrated in Fig. i. The modified system is identical to the structure illustrated in Fig. 3 with the exception that one or both oi the lines 43, 45 forming the T-junctions provide a high-attenuation wave path between the cavity resonators 5! and 53, effectively to prevent reaction therebetween which would disturb the phase relations of the enclosed fields. The T-junctions may he made to provide the required attenuation by employing a lossy dielectric 61, in one or both of said junction lines, such as conductive rubber, as indicated by the shaded dielectric in the T-junction line 45.

Thus the invention described comprises several embodiments and modifications of an improved balanced detector for microwave reception wherein proper phase relations are established in a waveguide, coaxial line or cavity reso nator structure in response to input signals derived from a pair of input lines.

What is claimed is:

1. A. balanced microwave detector for a pair of signal sources comprising a pair of cavity resonators, a pair of wave detecting means each disposed within one of said resonators, a first input signal source oppositely inductively coupled through both of said resonators to both of said detecting means for coupling signals thereto in phase opposition, a second input signal source inductively coupled in-phase through said resonators to both of said detecting means, and means for connecting said detecting means to a utilization circuit.

2. A. balanced microwave detector for ,a pair of signal sources comprising a pair of cavityresonators, a pair of wave detecting means each disposed within one of said resonators, a first input signal source, a first coupling line terminated in similarly phased coupling loops disposed Within said resonators for coupling signals in phase to both of said detecting means, a second input signal source, a second coupling line terminated in oppositely phased coupling loops disposed within said resonators for coupling signals in phase opposition to both of said detecting means, and means for connecting said detecting means to a utilization circuit.

3. Apparatus according to claim 2 including separate tuning means disposed within said resonators intermediate the ends thereof remote from said coupling loops and said wave detecting means for separately matching said detecting means at the operating microwave frequency.

4. Apparatus according to claim 2 including separate tuning means disposed within said resonators for separately matching said detecting means at the operating microwave frequency.

5. Apparatus according to claim 2 including wave attenuating means interposed in one of said coupling lines for preventing reaction between said cavity resonators.

6. Apparatus according to claim 2 including wave attenuating dielectric in one of said coupling lines for preventing reaction between said. cavity resonators.

RENE A. BRADEN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,413,939 Benware Jan. '7, 1947 2,420,892 McClellan May 20, 1947 2,547,378 Dicks Apr. 3, 1951 

